If a document is inserted without a provided _id value, then that field and value will be automatically generated for the inserted document before insertion.
This is true.
MongoDB ensures that each inserted document has a unique _id value.
There is maybe here an issue.
So _id is the field.
Field and value will be generated?
_id is the field, and the random _id after the : is the value.
But the field is never generated, it will be always _id, no way to change that.